(CNN) -- In an emotional open letter to the person responsible for her daughter's disappearance, the mother of missing Indiana University student Lauren Spierer pledges to never give up searching for answers.

"If you think for a minute her father and I are going to disappear, think again," Charlene Spierer writes in the letter, written on the three-month anniversary of Lauren Spierer's disappearance and released Saturday on the family's "Find Lauren" web site.

"We are just as determined today as we were day one. You should know that Lauren is precious to us. ... We are in this for the long haul. Do you think we are going to walk away without finding out the answers? Do you think we are going to rest until we find Lauren? We will not. You have no idea who you have taken from us. We will never give up."

Lauren Spierer, a 20-year-old sophomore, was last seen early June 3 after leaving a Bloomington, Indiana, sports bar with a male student. Surveillance camera footage from an alley between the two students' apartment buildings showed them walking toward the male student's building about 3 a.m. Spierer's keys were later found in the alley, but she has not been seen since.

One of the male student's neighbors reported seeing Spierer leave about 4:30 a.m., but police have said they found no sign of her after the 3 a.m. sighting on any of the surveillance video from the area between the two buildings. Police have not named any suspects..

In her letter, Charlene Spierer details the heartache her family has experienced since the disappearance.

"Much has happened over the past three months," she wrote. "I won't bore you with the details, if you've even gotten this far in reading this letter. I will say we have had the support and guidance of a multitude of people which has helped keep us going. For that generosity, we will be forever grateful."

She recalls being asked by a young relative, at a birthday celebration for her other daughter, Rebecca, whether the family would celebrate Lauren Spierer's birthday even if she was not there. "I didn't really know what to say," she writes. "Now I do. Of course we will celebrate Lauren's birthday."

What are you thinking? I would really like to know. I am waiting to hear from you.
--Charlene Spierer, in open letter to person responsible for her daughter's disappearance

She writes that as students returned to Indiana University for the fall semester, her family was packing her daughter's possessions and shipping them home. "Nineteen boxes," she said. "Can you imagine that so much promise would fit into 19 boxes? I won't go into the details of what it was like to pack Lauren's things. I will just tell you it was excruciating. Every moment was excruciating."

Charlene Spierer writes that she and her daughter share many things. "Today I can add to that list," she says -- the day that marks three months since the disappearance is also Charlene Spierer's birthday.

She recalls feeling "physically sick" upon learning her daughter was missing, calling hospitals in the hope that a Lauren Spierer or Jane Doe had been admitted. She immediately flew to Bloomington from New York, she writes, and her father drove her to the airport.

"Imagine a grandfather, driving his daughter to the airport on the way to discover what has happened to his granddaughter," she writes. "Imagine that. There were few words exchanged. There are no words."

"You are responsible for Lauren's disappearance," she writes. "You know where she is. You are guilty of a heinous crime."

She closes by asking the person, "What are you thinking? I would really like to know. I am waiting to hear from you."

At the end, she attaches a brief note to her daughter: "Dearest Lauren, we are loving you. Missing you. Always, Mom."

Lauren Spierer was last seen wearing black pants and a white tank top with a white shirt, according to her family's web site. She is 4 feet 11 inches tall and weighs 95 pounds and has blond hair and blue eyes.

Police searched a landfill outside of Bloomington last month in connection with Spierer's disappearance. Nothing was found, according to CNN affiliate WISH.

Attempts to reach the Spierer family Sunday were not immediately successful.